The present invention relates to apparatus for controlling the operation of hydraulic machines which can be used as pumps or motors, and more particularly to improvements in apparatus for adjusting a slide block or an analogous regulating member or device in a hydraulic pump or motor wherein the regulating member influences the throughput, the direction of fluid flow and/or other characteristics of the machine.
It is already known to change the position of the slide block in a hydraulic rotary piston pump or motor by means of a follow-up control wherein one or more plungers serve to change the eccentricity of the slide block with respect to the cylinder block to thereby change the strokes of pistons and/or the direction of fluid flow through the machine. The follow-up control is influenced by a control valve which responds to changes in one or more characteristics of fluid at the outlet of the machine. As a rule, the changes in pressure of fluid at the outlet of the machine are monitored by a flow restrictor and the control valve responds to changes in pressure differential at the opposite sides of the flow restrictor. The control valve can connect the chamber of a cylinder for the plunger of the follow-up control with a source of pressurized fluid or with a reservoir to thereby effect a change in the position of the plunger and slide block. The slide block influences the operation of the machine to thereby change the corresponding characteristic of the fluid at the outlet of the machine so that the pressure differential at the flow restrictor again assumes a value which is within the preselected range.
In certain presently known follow-up controls, the plunger is a differential piston having an axial bore for a reciprocable valve member. When the fluid pressure at the outlet of the machine exceeds a permissible value, such pressure opens a pressure relief valve in a conduit which connects the outlet of the machine with a reservoir and contains a flow restrictor. The pressure differential at the flow restrictor decreases and this results in displacement of the differential piston in a direction to reduce the fluid pressure at the outlet in response to appropriate adjustment of the slide block. As a rule, such opening of the relief valve initiates the establishment of a connection between the chamber of the cylinder for the differential piston and the reservoir so that the chamber can discharge fluid and such fluid allows a shifting of the differential piston for the purpose of reducing the pressure of fluid at the outlet of the machine. The valve member in the differential piston remains at a standstill and cooperates with the displaced differential piston to permit the flow of fluid through a channel which is machined into the differential piston and contains a throttle. Such fluid is maintained at an elevated pressure and flows into a chamber wherein it opposes a return movement of the differential piston. This causes considerable losses in fluid and adversely affects the efficiency of the machine. Proper operation is assured only by appropriate dimensioning of the throttle in the channel of the differential piston. However, the throttle invariably delays the adjustment of follow-up control so that the inertia of the apparatus is considerable and the adjustment is completed with a substantial delay.